The removal of sulfur compounds from crude oil and its fractions has been of significant importance for several decades, but has become even more important in recent years due to tightening environmental legislation. While much of the prior art focuses on the desulfurization of individual crude oil fractions, a large segment of the art has addressed processes for the hydroprocessing of whole crude oil. The majority of the interest in recent years has focused on the upgrading of very heavy crude oil (i.e., API gravity<20), shale and tar-sands, to produce light sweet synthetic crudes. One major driving force for these processes is the demand for light crude oils in refineries and the low value of highly viscous feedstocks. Furthermore, refinery demands are shifting from high sulfur fuel oils to low- and ultra-low sulfur products, i.e., products containing about 1 wt % (LSFO) and about 0.5 wt % (ULSFO). Therefore, the ability to produce LSFO or ULSFO, instead of high sulfur fuel oils, is highly advantageous and desired.
One major technical challenge posed when hydrotreating heavy oil fractions or whole crude is the effect of small concentrations of contaminants, such as for example, organic nickel and vanadium compounds. These organometallic compounds and others have been proven to reduce the activity or lifetime of hydrotreating catalysts.
Another major challenge faced during the processing of whole crude oil is that the concentration of coke precursors can generally be very high. These coke precursors, such as for example, asphaltenic plates, can reduce the activity or lifetime of hydro-desulfurization (HDS) catalysts. This results in a decreased performance of a conventional process over time, thus requiring more frequent addition or replacement of catalyst to ensure continued operation. Catalyst replacement can be both costly and time consuming, thereby reducing the overall economic feasibility of the process.
Generally, deactivation of catalyst within a hydroprocessing unit typically occurs by one of two primary mechanisms: (1) metal deposition and (2) coke formation. For each mechanism, increasing the operating temperature of the hydroprocessing unit can help maintain catalyst performance; however, all process units have maximum temperature limits based upon the metallurgy of the process unit. These maximum temperatures limit the amount of time a catalyst can operate before requiring catalyst replacement, typically by either the regeneration of spent catalyst or the addition of fresh catalyst. Furthermore, the replacement of spent catalyst with fresh catalyst can require the complete shutdown of a process unit in order to unload the deactivated spent catalyst and load fresh catalyst into the unit. This process unit downtime reduces the on-stream time and negatively impacts the economics of the process.